The present invention relates to an electric current detecting circuit, and more specifically relates to a particular electric current detecting circuit of the type having an output stage in the form of a H bridge circuit composed of four field effect transistors and being contained in a semiconductor integrated circuit of motor driver which is used in a floppy disk apparatus and a hard disk apparatus.
As shown in FIG. 1, such type of the conventional electric current limiting circuit is comprised of a H bridge circuit composed of four MOS transistors 5-8 and a resistor 25. Namely, as shown in FIG. 1, there is provided a control circuit 11 for feeding an output to first and second internal input terminals 12 and 13 in response to a signal from an input terminal 1 and further for controlling concurrently output voltage levels of the first and second internal input terminals 12 and 13 according to a signal level of an electric current detecting input terminal 14. The first MOS transistor 5 has a drain connected to the electric current detecting terminal 14, a source connected to a first output terminal 9 and a gate connected to the internal input terminal 12. The second MOS transistor 6 has a drain connected to the output terminal 9, a source connected to a ground terminal 4 and a gate connected to the internal input terminal 13. The third MOS transistor 7 has a drain connected to the electric current detecting terminal 14, a source connected to a second output terminal 10 and a gate connected to the internal input terminal 13. The fourth MOS transistor 8 has a drain connected to the output terminal 10, a source connected to the ground terminal 4 and a gate connected to the internal input terminal 12. The electric current detecting terminal 14 is connected to a power supply terminal 3 through the electric current detecting resistor 25. The FIG. 1 circuit operates to detect an electric current, flowing through the output buffer H bridge circuit, in the form of a voltage falling across the electric current detecting resistor 25 so as to control the voltages at the internal input terminals 12 and 13 according to the detected level to thereby regulate the electric current flowing through the output buffer H bridge circuit.
Since the above described conventional circuit operates to detect the electric current flowing through the output buffer H bridge circuit by effecting voltage-conversion through the electric current detecting resistor, this resistor consumes a great amount of electric power to thereby hinder the reduction of power consumption to a certain degree. Further, since a power resistor is externally needed, it is impossible to scale down a size of a printed circuit board etc.